fix(theme): add gray to theme (#914)

fix #912

* Add `gray` to theme

zellij-utils/src/input/theme.rs

@@ -20,6 +20,7 @@ pub struct PaletteFromYaml {
     pub fg: PaletteColorFromYaml,
     pub bg: PaletteColorFromYaml,
     pub black: PaletteColorFromYaml,
+    pub gray: PaletteColorFromYaml,
     pub red: PaletteColorFromYaml,
     pub green: PaletteColorFromYaml,
     pub yellow: PaletteColorFromYaml,
@@ -79,6 +80,7 @@ impl From<PaletteFromYaml> for Palette {
             fg: yaml.fg.into(),
             bg: yaml.fg.into(),
             black: yaml.black.into(),
+            gray: yaml.gray.into(),
             red: yaml.red.into(),
             green: yaml.green.into(),
             yellow: yaml.yellow.into(),